Cytokines are chemicals produced by one cell to affect a response in another cell. Cytokines are secreted proteins, found in the extracellular environment, which interact with specific target cells to communicate information regarding the status of the organism. In this way, cytokines control the survival, growth, and differentiation of cells by eliciting an appropriate biological response in the target tissue. For example, cytokines are produced in response to a microbial infection and trigger increased white blood cell production, function, and chemotaxis. Cytokines may be constitutively produced to maintain a steady state, such as continued cell survival and selection within the nervous system. Examples of cytokines include growth factors, interleukins, and interferons.
Cytokines interact with a target through receptors expressed on the surface of the responsive cell. Cytokines bind with hemopoietin receptors, receptor kinases, and tumor necrosis factor (TNF)/nerve growth factor (NGF) receptors by bringing together two receptor subunits. This dimerization of receptor subunits transmits a signal through the plasma membrane to the cell cytoplasm. In the case of protein kinase receptors, such as the receptors for epidermal growth factor (EGF) and insulin, the juxtaposition of the two receptor subunit cytoplasmic domains activates their intrinsic tyrosine kinase activity. As a result, the subunits phosphorylate each other. The resulting phosphorylated tyrosine residues then interact with cytoplasmic proteins containing src homology 2 (SH2) domains. SH2-containing proteins that interact with phosphorylated receptor molecules include phosphatidylinositol 3'-kinase, src kinase family members, GRB2, and shc. These SH2 containing proteins are often associated with other cytoplasmic proteins, such as members of the small, monomeric GTP-binding protein families Ras and Rho, and phosphatases, such as the phosphotyrosine phosphatase SHP-2. The signaling complexes formed by these interactions can initiate signal cascades, such as the kinase cascade involving raf and mitogen activated protein (MAP) kinase, which result in transcriptional regulation and cytoskeleton reorganization. Hemopoietin and TNF/NGF receptors, though they have no intrinsic kinase activity, still activate many of the same signal cascades within responding cells.
Many of the kinases involved in cytokine signaling cascades were first identified as products of oncogenes in cancer cells in which kinase activation was no longer subject to normal cellular controls. In fact, about one third of the known oncogenes encode protein kinases. Furthermore, cellular transformation (oncogenesis) is often accompanied by increased tyrosine phosphorylation activity (Carbonneau, H. and Tonks, N. K. (1992) Annu. Rev. Cell Biol. 8:463-93). Thus, the cell must have regulatory systems which keep the cytokine signaling cascades under appropriate control.
Eps8 is a protein which associates with and is phosphorylated by the EGF receptor. Human tumor cell lines contain high constitutive levels of tyrosine-phosphorylated Eps8, and overexpression of Eps8 in NIH3T3 cells expressing the EGF receptor (EGFR) leads to an enhanced mitogenic response and cell overgrowth (Provenzano, C. et al. (1998) Exp. Cell Res. 242:186-200). A family of molecules, which include ABI (Abl interactor protein)-1 and ABI-2/e3B1, interact with tyrosine kinases, such as the src-like kinase Abl, and Eps8. Overexpression of ABI-2/e3B1 in NIH3T3 cells expressing EGFR inhibits the mitogenic response and cell growth. Thus, the ABI family of proteins function as negative regulators of cytokine signaling (Ziemnicka-Kotula, D. et al. (1998) J. Biol. Chem. 273:13681-13692).
The SH2-containing phosphotyrosine phosphatases, SHP-1 and SHP-2, are involved in cytokine signaling. SHP-1, the hemopoietic cell phosphatase, is a potent inhibitor of signaling, whereas SHP-2 is a positive signal transducer for several cytokines. A family of transmembrane glycoproteins, called SIRPs (signal regulatory proteins), are substrates of tyrosine kinases. Phosphorylated SIRPs bind to SHP-2 and have a negative effect on cell response induced by cytokines, including an inhibition of growth factor-induced DNA synthesis. This inhibition correlates with reduced MAP kinase activation in SIRP-transfected NIH3T3 cells stimulated with insulin or EGF. SIRP overexpression also suppressed transformation of NIH3T3 cells by a retrovirus carrying the v-fms oncogene (Kharitonenkov, A. et al. (1997) Nature 386:181-186).
The discovery of new cytokine signal regulators and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of cell proliferative and immune disorders.